Just like Grim, Junior has the apperance of a skeleton, but smaller. He has brown hair on the top of his forehead, which could be from his unknown father. He also has his sister's right eye in his right eye socket. In Down Below, Junior wears a black hooded sweat-shirt, pants, athletic shoes, and fingerless gloves. In his human form, Junior has tanish skin and the small tuft of brown hair has grown to all the way to his occiput. His right eye is light grey, and his left eye is light blue, and he still wearing his usual black attire.
Though he is expected to become the new Reaper one day, as his father did before him, Junior couldn't care less. Instead, he wishes to have a normal life, possibly in the human world. This interest in mortals was kindled by his discovery of his father's diary, in which Grim had recorded all his adventures with Billy and Mandy. Grim Jr. only has his sister and his parents for company, as well as a few of his relatives on occasion. Grim Jr. has focused his time on activities like basketball, baseball, skateboarding, trading cards, and breakdancing, neglecting his studies as a result. Ms. Helga, the tutor his mother hired for her children, is displeased with this. Grim Jr. is extremely jealous of his sister, Minnie, though he does care for her regardless. This is because his sister is both naturally popular and talented, while Junior isn't. Another source of contention is how Junior's godfather, Nergal Jr., spends more time with Minnie than Junior. Junior idolizes Nergal Jr. and feels slighted by the attention Minnie receives from Nergal Jr. Nergal Jr.'s attention to Minnie is likely due to the fact that she's actually his child, not Grim's.
Although he hasn't seemed to have inherited his fathers' reaping powers, he does have a demonic abilities thanks to the gray eye that came from his sister, Minnie. He seems to be able to use the same black substance with glowing orbs that Minnie used to have, forming it around his body to make a weapon , or multiple coverings he cane form around his body from transport. the possibilities seem to be endless. but don't be fooled. Grim Jr. doesn't seemed to have mastered this yet, and his sister can easily take control by using a type of language to 'speak' to the goo. He also seems to be able to live with detached limbs, this ability was shown when Mimi ripped his skull head off, along with his spine. This does seem to hurt him when someone cuts the black 'goo', though it quickly regenerates.
In Grim TalesEdit
When Junior was born, he was delivered through abortion. He has the appearance of a skeleton, like his father, but later acquires the a human form due to his sister's powers that are transfered over to him. His sister, Minnie, was born one year later, whom he became extremely jealous of due to her faultless reputation among everyone. Like her he is the Surrogate child of Grim, which they are full aware of since Grim is incapable of creating life. Nine years later, Junior, along with the rest of his family, attended a Halloween party with his "uncle" Jack. After a display that "ridicules him in the eyes of others", Junior storms off, furious with his sister.
Soon after, he meets Lock, Shock and Barrel and they ask him to join their gang. In exchange, he tells his family story (after kidnapping him). Once he finishes telling the story about how his parents got together, he is knocked out by a creature called the Oogie Boogie Man. He was then taking hostage by Oogie in order for Oogie to pry him open and take any potential Reaper powers, but Minnie charges in to save him, using her own demonic (Nergal) powers.
Rather than thanking Minnie for saving his life, he tells her off and turns to leave her, disheartened and upset by her brother's rejection. When Junior turns his back, Oogie Boogie grabs hold of Minnie using the large Pumpkinator and she is dismembered in front of his eyes. Once he reaches her side, she painfully removes her right eye and hands it to him, after which, he meets Nergal (Sr.), who explains how Minnie's powers has been transferred to him (Junior).
Junior defeats the Pumpkinator and saves Minnie's soul, but an angel arrives, telling Junior he make take Minnie to heaven for her purity and good heart. Junior charges to attack, getting his hand chopped off and failing to stop them, and after Minnie and the angel depart, Minnie demonic powers consumes the Pumpkinator's fallen husk and starts to devour the Underworld, leaving Junior's body as the host. After Junior is saved by Grim and Clockwork, he is taken away by Clockwork to the eight levels of the Underworld as to search for his sister.
Junior then meets the angel who took Minnie away, who is apparantly also searching for Minnie. It turns out that Minnie is kidnapped by Him. Soon afterward, he meets Mimi and Jeff, who offers both Junior and the angel to stay in his home until the blizzard is over. Both the angel and Junior are betrayed by Jeff and Mimi, who turn out to be working with Him - Mimi is revealed as him's daughter and soon transforms into a female look-alike. After offering Junior the chance at his sister and great power, Junior rejects the offer and tries to leave with Minnie, angering Him and causing Jeff to betray him. After engaging into a fight with him, junior tells the angel to take Minnie to "somewhere safe.". When Him is about kill Jeff for his betrayel, Mimi saves the boys and takes them to a place called Limbo.
After escaping him, Jeff explains that he will be back to punish them and not to worry, just as Clockwork arrives and takes Junior back (after trying to have him save Jeff and Mimi) to Halloween Town. After coming back, Junior is soon encountered by the Nergal beast, proceeding to try and absorb him again. Minnie arrives (who declines her a chance of going to heaven), in her angelic form to help Junior fromthe Nergal beast. When Junior and Minnie is eaten by the power, Junior meets Nergal once again, but in a different form. Both save Minnie and both Minnie and Nergal talk in their race's ancient language in order to stop the monster rampage. The monster starts losing control over itself and being re-controlled by Minne. After the smoke clears out, hit turns out neither Nergal (Sr.) nor Minnie's body survive the attack by Dark Danny. In the aftermath of the battle, they create a ragdoll like body for Minnie, where Junior proceeds to transfer her soul into it, thus having a closer bond with his sister and her Nergal powers still residing within him.
Afterwards, Mimi was ordered to raid Mandy's vault for the Hand. She did this so she could save Jeff. She defeated the guards easily and on her way to the vault with a map and flashlight, she met Junior and Minnie. Junior transformed into his human form, asking Mimi of why is she here. She was simply flossing her teeth, but became angered when she heard Junior spoke of Jeff. She snapped the floss between her teeth, spitting out blood. She went up close, face-to-face with Junior, which happened last time before he left. This action caused Junior to blush, and Minnie with a hostile face. Mimi slapped Junior with a toaster, and then she pulled his head out, along with his spine. Minnie ordered Mimi to release her half brother's head, but before doing so of finishing her sentence, Mimi threw an iron in Minnie's face. Minnie was angered, causing to fight with a Nergal sword she had summoned. Mimi dropped Junior's head. Junior saw that Mimi wears pink underwear. Junior's body, the "goo" grabbed Mimi, but she seemed to enjoy it. She trapped Junior and Minnie to the ground, hammering the "goo" to the ground.
Mimi fought off Grim's dogs easily. She pulled up her skirt, locating an item to hack into the vault. She drank some mouthwash, then she spotted a tricycle with keen hope. But Mandy came into the room, armed in guns and almost naked (in pink lingeries and a transluctinate pink robe.) Mimi fought back with angry birds. An angry bird egg landed on Mandy's robe, melting away. She removed it as Mimi hid behind an artifact, eating some sardines. She shot bullets at Mimi.
Meanwhile, Grim spotted Junior and Minnie were stuck together, but couldn't be freed. Grim cut off the nergal "goo", and he spotted a copy of the map to the vault. He came in, seeing Mandy in her pink lingeries, commenting had she lost weight. Then Junior and Minnie came in, seeing Mandy in her pink lingeries, commenting had she gained weight. Junior tried to explain about his short friendship with Mimi, but Mandy revealed the Horror Hand. Everybody was summoned back, finding themselves in the ruins of Megaville, the city of heroes.
Mimi broke down, after seeing her mother, Blossom (from the Powerpuff Girls) torn apart from each other. She let out a scream of pain and sadness. That scream shattered the glass to the collection, making all the items falling out of place. This caused Mandy to become mad, and removed Mimi's demonic side away. Mimi became delicate and weak. Mandy ordered Lord Pain to take Mimi to the dungeon, but Junior protected Mimi. Mandy warned Junior to move, but he still didn't move. She beated up her son up, giving him once more to move, but he still showd defiance and transformed in his nergal form. Mandy admitted that she is a bit impressed with Junior's defiance. She told him that she thought Junior "was as spineless as his father,". She commented Junior to keep that thing out of her vault or else, Mimi's head will be added to her collection.
Junior carried Mimi, but Minnie reveal to have a disapproving face.
On Sora's TeamEdit
When Sora visited Junior's world, he was happy to see his godfather Nergal Jr. and friends Lock, Shock, and Barrel again. He quickly wanted to join the team and already became one of the top leaders of the team after her rescued them from almost being lunch by Oogie Boogie.